Stem borer (Coleoptera: Mordellidae) in Calamagrostis

In early November 2025 I found a mordellid larva in its tunnel in the lower culm of an ornamental bunchgrass from a garden bed in my yard. The host plant appeared to be a Calamagrostis, probably feather reed grass (C. x acutiflora).

The larva’s tunneling was most visible in the lowermost two or three internodes, betrayed by frass accumulation and signs of damage to the inner wall of the culm. A thin tunnel proceeded from the second-to-lowest internode through the node to the lowest internode, and the top of this internode was filled with a ~8-10mm rod of compacted frass. The larva dwelled below this frass rod near the base of the internode, facing up. It was a pale creamy color, whitish with a slightly yellowish or tan hue.